The present invention relates to spatial measurements of indicia on a moving surface and, in particular, to utilization of such measurements in arts such as the printing arts.
Known printing systems employ a number of printing cylinders that successively print on a moving web differently colored patterns. An important consideration with such printing techniques is accurate registration of the patterns, which may be printed at a considerable distance from each other. It is known to print cross hairs at each printing cylinder to allow an operator to register visually the several patterns by manually adjusting the web path until all of the cross hairs register. Such manual adjustment is unsuitable for high speed printing operations where variations may occur too quickly to allow precise registration.
It is also known to automatically control registration of successively printed patterns on a web by employing photoelectric sensors that detect passage of a predetermined mark on the web. By employing a pair of such photoelectric sensors and a timing clock, the difference in time between arrival of the indicia can be used as an error signal. A disadvantage with measuring the time difference between two marks is that the resulting error signal is a function of web speed. Accordingly, any feedback loop employing this error signal will have its gain affected by the speed of the web, thereby fostering either a slow response or over-shoot. Another disadvantage with this known arrangement is the necessity of printing the indicia on separate tracks and in longitudinal alignment (alternatively, the photoelectric sensors must be positioned to account for any intentional misalignment). This restriction may be a serious disadvantage where a pattern ought to be printed right up to the edge of the web. In this case, there is either no room for the indicia or additional web material must be used and then stripped away as wastage merely for the purpose of registration.
Measuring the displacement between indicia with a conventional shaft encoder driven by a printing cylinder has serious disadvantages. Known shaft encoders do not economically provide sufficient resolution to permit the precise registration required in the printing trades. Using a tachometer device as a transducer for a printing cylinder to derive displacement also has serious disadvantages. Such a tachometer device could convert a speed signal into a distance increment by a voltage controlled oscillator. However, a controlled oscillator, operating typically by ramping until it reaches a controlled threshold value, has an unacceptable linearity since its retrace time is finite and non-linearly affects the frequency of operation.
Accordingly, there is a need for an accurate system for measuring displacement of indicia on a moving surface that overcomes the limitations of conventional transducers.